llvm dev - Nov 2018 - LinkOnceODRLinkage behavior on Windows

Hi,

I'm compiling two files with clang 6.0: testA.c and testB.c.

On both files I am running a custom compiler IR-level pass that inserts a
zero-initialized global variable called globalVarTest. I set the linkage of such
global variable to be LinkOnceODRLinkage.

Now, when I link testA.c and testB.c together (that is, the object files
obtained by compiling them separately), the linker complains that there are
multiple definitions of globalVarTest.

Isn't LinkOnceODRLinkage supposed to tell the linker that if both
definitions match, the symbols is the same and it can therefore pick any of the
two definitions when building the final executable?

The same procedure works on Linux. Is this a limitation of the Windows linker? I
have been able to achieve the same effect using COMDAT sections but I wonder if
that's an appropriate thing to do or if there's a better mechanism to
achieve what I want.

Thanks,
Daniele
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Yes, apply comdats to get the old behavior. This was recently discussed in
the swift tracker: https://github.com/apple/swift/pull/20059
It happened back in 2015, but I don't have any better reference material,
unfortunately.

On Fri, Nov 16, 2018 at 11:54 AM Daniele Vettorel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I'm compiling two files with clang 6.0: testA.c and testB.c.
>
> On both files I am running a custom compiler IR-level pass that inserts a
> zero-initialized global variable called globalVarTest. I set the linkage of
> such global variable to be LinkOnceODRLinkage.
>
> Now, when I link testA.c and testB.c together (that is, the object files
> obtained by compiling them separately), the linker complains that there are
> multiple definitions of globalVarTest.
>
> Isn't LinkOnceODRLinkage supposed to tell the linker that if both
> definitions match, the symbols is the same and it can therefore pick any of
> the two definitions when building the final executable?
>
> The same procedure works on Linux. Is this a limitation of the Windows
> linker? I have been able to achieve the same effect using COMDAT sections
> but I wonder if that's an appropriate thing to do or if there's a
better
> mechanism to achieve what I want.
>
> Thanks,
> Daniele
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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